Experimental study of the hygromechanical behaviour of a historic painting on wooden panel : devices and measurement techniques

Abstract

This article describes the experimental devices and the processes used to study the hygromechanical behaviour of a historic painted wooden panel (La Sainte Trinité couronnant la vierge, 1516, anonymous, Fabre Museum of Montpellier). A climate showcase was designed for in-museum use, with two glasses allowing visitors as well as scientists to observe both sides simultaneously. The evaluation of the hygroscopic behaviour was done by measuring relative humidity (RH), temperature and variations of panel weight. Variations of panel shape, strain and curvature were observed by two methods: locally with three pairs of displacement transducers fixed on the rear face, and over the entire panel surface by a non-contact optical technique, stereo mark tracking, used simultaneously on both faces. The correlation between local and whole-field measurements was very good. Continuous monitoring (several data per hour over three years) was required to observe the behaviour of the panel during imposed climate variations. The first test, detailed in this paper, was performed before frame removal and panel restoration. It consisted in a stabilization of the panel at 52% RH during 2 months and an increase to 63% RH during 3 months. The small amount of total mass variations, from 31.980 kg to 31.910 kg at 52% RH and 32.088 kg at 63% RH, could not be explained without taking into account the phenomenon of sorption hysteresis. The whole field relief measurement exhibited a concave shape with a maximum amplitude of 16 mm. It showed that shape deformation, ie out-of-plane displacement, was convex bending with a maximum deflection of 1.5 mm and a maximum strain of 0.1 %. Comparable local data had been obtained by the optical and DKs methods. DKs results were ten times more accurate than optical ones. The horizontal strain exhibited a global shrinkage during initial stabilization and a global swelling during RH increase. Heterogeneity of the strain field can be related to the cracks observed on the back face, and to the glued crossbeams. Assuming a linear relationship between swelling and moisture content, we expected, for a 0.6% increase of moisture content, a radial strain of 0.10% and a tangential strain of 0.22%. These results are higher than the ones measured, which can be explained by the cross bars glued on the boards and the cracks. This comparison will be continued for the next stages (crossbars removal, panel restoration).

Heat and Moisture Induced Stress and Strain in Wooden Artefacts and Elements in Heritage Buildings: A Review

“…The DK is suitable for describing panel behaviour, its deformations, i.e., both in-plane and out-of-plane, and their evolution over time. As in Dupre et al [63], it is possible to directly monitor local strain and the cupping angle with a high accuracy. Further, the author combined the DK's techniques with two stereo-image acquisition, complementary techniques that allow a whole analysis of the painting, i.e., front and rear of the panel, to be obtained.…”

“…In this context, non-destructive technique (NDT) and advanced mathematical modelling are very often employed [62]. Furthermore, there are useful approaches that combine different methods and techniques to capture all the factors that affect the thermo-hygro-mechanical behaviour, such as moisture content, wood density, and growth ring orientation, among others [26,63]. Nonetheless, this section overviews experimental and theoretical techniques by focusing on measuring moisture-induced stress and dimensional changes in wooden artefacts, as well as by avoiding the description of complementary techniques, which aim at measuring influencing parameters, such as moisture content, by using a vacuum microbalance.…”

Investigating the accuracy of digital image correlation in monitoring strain fields across historical tapestries

“…The use of highly accurate techniques of digital image correlation (DIC) [1] for full-field strain measurement and experimental analysis of fluid, biomechanical and mechanical structures and components is very common and well established. Because DIC is a noncontact surface strain measurement technique, it is commonly used in monitoring full-field strain and deformation across delicate heritage paintings [2][3][4] and historical tapestries. [5][6][7][8][9][10] The previous studies using DIC for strain monitoring of historical tapestries focused on the measurement of strain distributions across tapestries under self-weight loading conditions, using the tapestry's inherent image to track deformation; the usual application of a speckle pattern [11] on the specimen surface is not feasible with historic artefacts.…”

“…Monitoring strain and deformation across panel paintings using their natural texture as DIC tracking information has been found to be less effective. [2] To improve the performance of DIC measurement across panel paintings, removable mark trackers are placed strategically across the painting panel surface. [2,4] Due to the physical condition, size, weight and position of historical tapestries in historic houses and museums, using such mark trackers for DIC in situ strain monitoring is not feasible and the only tracking information is often the fabrics' inherent texture.…”

“…[2] To improve the performance of DIC measurement across panel paintings, removable mark trackers are placed strategically across the painting panel surface. [2,4] Due to the physical condition, size, weight and position of historical tapestries in historic houses and museums, using such mark trackers for DIC in situ strain monitoring is not feasible and the only tracking information is often the fabrics' inherent texture. Extensive research on DIC performance assessment techniques [11][12][13][14][15][16] has shown that in the context of normal materials and structures, the quality, density, size and randomness of speckle pattern applied on the test sample surface are fundamental factors that determine the accuracy and reliability of the DIC measurement results.…”